Gary"s Student, you do not have good news , but maybe you don't understand
the question being asked. Like TREND, FORECAST uses all the data points in
the array and finds a best fit considering the entire array of data points.
Suppose A1:A4 = 1,2,2,1 (known y values) and B1:B4 = 1,2,3,4 (known x
values). FORECAST(2.5,A1:A4,B1:B4) returns a y value of 1.5. The function
everybody and their cousin wants - INTERPOLATE(2.5,A1:A4,B1:B4) - would
return a y value of 2.

Put these numbers in the spreadsheet and try it. Hopefully this makes sense
to you so you'll spreading wrong information about interpolation on this
board. I really wish you were right, though.

"Gary''s Student" wrote:

I have good news.

The FORECAST() function, which is usually used to extrapolate outside a
range of known points also interpolates for a point between two known points.
Checkout help for the function syntax.

(this is an un-advertised feature of the function)
--
Gary''s Student


"tskoglund" wrote:

Given a series of x and y data, how can I interpolate to find y given a value
of x based only on a line between the two adjacent points in the data series?
This would be like the TREND() function, only I don't want regression of the
entire data series, just the (x,y) data points immediately above and below
the input x value.

This seems such fundamental and essential function that I've been
exasperated for years that Excel doesn't have it or directions how to find
it. I realize that Excel calculates these individual lines every time it
connects the dots in a graph, but not having a function to achieve the same
result on a worksheet is puzzling. Am I missing something?